1. Fie1d of the Invention
This invention relates to an internal combustion engine, and more particularly to an internal combustion engine having a pair of intake valves and a pair of exhaust valves for each cylinder.
2. Description of the Prior Art
There has been put into practice an internal combustion engine in which each cylinder is provided with a pair of intake valves and a pair of exhaust ports in order to improve the volumetric efficiency and the scavenging efficiency, thereby improving combustion in the combustion chamber and increasing the engine output power. Such an engine having a pair of intake valves and a pair of exhaust valves will be referred to as "four-valve engine", hereinbelow. The four-valve engine, however, involves the following difficulties. That is, since four valves must be driven for each combustion chamber in the four-valve engine, the space around the cylinder head is crowded and accordingly, difficulties arises regarding the position of the spark plug (the fuel injection nozzle in the case of a diesel engine). In order to obtain good ignition of the air-fuel mixture and good propagation of flame in the combustion chamber, the spark plug or the fuel injection nozzle should be centrally located in the combustion chamber. A particular problem is that in the case of a single overhead camshaft four-valve engine in which the overhead camshaft is centered over the combustion chambers, mounting and demounting of the spark plug or the fuel injection nozzle is interfered with by the overhead camshaft.
In Japanese Unexamined Patent Publication No. 57(1982)-102506, there is disclosed a single overhead camshaft four-valve engine in which the spark plug is obliquely supported on the exhaust port side on which a relatively large amount of room remains and only the igniting tip is centrally located in the combustion chamber. As shown in FIGS. 10 and 11, the engine has a pair of intake valves 2 and a pair of exhaust valves 3 for each combustion chamber 1, the intake valves 2 and the exhaust valves 3 being arranged at the respective corners of a rectangle. A single overhead camshaft 4 is centrally positioned over the combustion chamber 1. A spark plug 5 is obliquely disposed between the exhaust valves 3 so that its igniting tip 5a is centrally located in the combustion chamber 1 and its upper end 5b is directed obliquely upward beside the camshaft 4. This arrangement permits the igniting tip 5a of the spark plug 5 to be centrally located in the combustion chamber 1 so as to obtain good ignition of the air-fuel mixture, and prevents interference of the spark plug 5 with the camshaft 4. However, with this arrangement, the rocker arm shaft 7 for pivotally supporting the rocker arms 6 for driving the exhaust valves 3 in response to rotation of the camshaft 4 must be separated for each rocker arm 6 in order to give access to the spark plug 5 because, if the rocker arms 6 are supported on a single shaft, the upper end 5b of the spark plug 5 extending substantially in parallel to the valve stems of the exhaust valves 3 is positioned immediately below the rocker arm shaft between the rocker arms 6 for each combustion chamber 1 so that the rocker arm shaft interferes with mounting and demounting of the spark plug 5. Therefore, a pair of rocker arm shafts are required for each combustion chamber. This significantly complicates the structure for mounting the rocker arm shafts especially in the case of a multiple cylinder engine.